1. Field of the Invention
This invention relates generally to gas sensors, and more particularly, to solid state sensors that can selectively detect the presence of hydrogen.
2. Description of the Related Art
A device that can detect the concentration of hydrogen in the presence of other gases would have multiple uses in, inter alia, the transportation industry. For example, in the space program, a mass spectrometer is employed to detect the presence of hydrogen in and around the space shuttle. This known device suffers from the disadvantage that, while it can detect the presence of hydrogen, it is incapable of identifying the source, or location of hydrogen leakage. There is therefore, a need for small, relatively inexpensive sensors, that can be placed in multiple locations for detecting a source of hydrogen gas emission.
As another example, the automotive industry has been developing new power sources, such as a hydrogen combustion engine and a hydrogen fuel cell. The safe use of these new power sources will require hydrogen sensors that can operate over a broad range of temperatures, pressures, and gas compositions.
Although there has been some effort expended to develop hydrogen sensors in the past, most, if not all, known hydrogen sensors have not entered commercial production because they have failed to meet all of the required parameters. Hydrogen can be detected readily in an environment that contains only hydrogen. However, it is considerably more difficult, with current technology, to detect hydrogen when it is mixed with other gases. Furthermore, with current technology, if a sensor is optimized to overcome the selectivity problem, temperature and pressure requirements are not satisfied. There is a need, therefore, for a sensor that overcomes all the present problems of selectivity, temperature, and pressure, whereby the sensor would be made usable in a realistic environment.
It is known that catalytic metals can be used as gates for gas sensitive field effect devices, such as transistors, capacitors, diodes, and the like. Known devices include metal-insulator-semiconductor (MIS) or metal-oxide-semiconductor (MOS) structures. Gas sensitivity occurs because reaction intermediaries give rise to electrical phenomena at the metal-insulator or metal-semiconductor boundaries. In a hydrogen sensor, for example, molecular hydrogen dissociates at the catalytic metal electrode surface and the hydrogen atoms produced diffuse through the electrode and are adsorbed at the electrode/insulator interface. The dipole moment of the adsorbed atoms produce a detectable change in threshold voltage of the device, thereby giving an indicate of the concentration of hydrogen in the gas to which the device is exposed.
The prior art has determined that palladium is the ideal catalyst for hydrogen diffusion. Hydrogen travel time within a palladium thin film is sufficiently small for this catalyst to be used as a selective “membrane.” Palladium (Pd) may provide acceptable selectivity, but when pure Pd films are used, other problems are typically encountered. For example, below 300° C., Pd undergoes an a-b phase transformation in the presence of a high concentration of hydrogen. Also, contraction and expansion of a Pd film leads to embrittlement and eventual fracture of the metal.
To overcome these problems, the prior art has suggested using Pd in combination with other materials. In particular, the prior art has explored the use of Pd/Ni alloy and Pd/Group V/Pd membranes. However, such membranes have exhibited temperature range limitations. For example, if these known devices are used at high temperatures, the membrane layers will melt into each other, preventing hydrogen from diffusing therethrough. Moreover, electron beam evaporation is used in the construction of the known alloys, requiring a substrate that also melts if used at high temperatures. In one effort to overcome these temperature limitations, a polycrystalline diamond film was applied over a Pd thin film by plasma-enhanced chemical vapor deposition. The temperature problem was not overcome since this device was useful only to 200° C.
It is additionally a problem that palladium is a good catalyst for many reactions, and therefore, poisoning occurs on the palladium surface. By “poisoning,” it is meant that other gases also adsorb at the palladium surface, closing the pores necessary for the diffusion of hydrogen. Some of the many adverse gases in this scenario are oxygen (O2), and particularly carbon monoxide (CO). The presence of O2 at the Pd surface results in dissociation into single oxygen atoms, which then react in the presence of hydrogen ions to form water. Fortunately, the water evaporates and frees the Pd sites. In fact, in order to purge palladium films from hydrogen, a flow of oxygen gas is supplied on the surface of the device, and vice versa, to purge a Pd film from oxygen poisoning, hydrogen gas is used. On the other hand, on a Pd surface, CO does not react easily with other elements, and therefore, CO poisoning is a significant problem in the art.
There is, therefore, a need for a hydrogen sensor that can operate over extended ranges of temperature and pressure, and in the presence of multiple gases and contaminants.